1. Field of the Invention
The present invention relates to an image sensing device and imaging system.
2. Description of the Related Art
Some recent imaging systems such as a digital camera and a digital video camera adopt an image sensing device, such as a CMOS sensor, which has an active element in a pixel and allows formation of peripheral circuits on a single chip.
The CMOS sensor includes a pixel array. In the pixel array, a plurality of pixels are arrayed in directions along rows and columns. Each pixel includes a photodiode, a transfer MOS transistor, a floating diffusion (FD), a reset MOS transistor, and an amplification MOS transistor.
The photodiode generates and stores charges corresponding to light. The transfer MOS transistor transfers the charges generated in the photodiode to the FD. The FD converts the transferred charges into a voltage. The reset MOS transistor resets the FD. The amplification MOS transistor outputs an N signal corresponding to the voltage of the FD to a column signal line while the reset MOS transistor resets the FD. The amplification MOS transistor outputs an S signal corresponding to the voltage of the FD to the column signal line while the transfer MOS transistor transfers the charges of the photodiode to the FD.
The threshold voltage of the amplification MOS transistor varies between pixels. Further, the FD generates kTC noise (thermal noise) when the reset MOS transistor resets the FD. As a result, fixed pattern noise is mixed in N and S signals.
To reduce fixed pattern noise, in a technique disclosed in Japanese Patent Laid-Open No. 2005-223860, a CDS circuit on each column performs CDS processing to calculate the difference between N and S signals transferred from a pixel array via a column signal line, generating an image signal free from fixed pattern noise. In the technique disclosed in Japanese Patent Laid-Open No. 2005-223860, N and S signals are directly read out to calculate and hold the difference between them. In this technique, when the difference level is low, the S/N ratio of an obtained image signal may decrease.
To increase an S/N ratio, in a technique disclosed in Japanese Patent Laid-Open No. 2005-217771, a clamp capacitor and an operational amplifier on each column execute CDS processing to amplify the difference between N and S signals transferred from a pixel array via a column signal line. According to the technique disclosed in Japanese Patent Laid-Open No. 2005-217771, because the difference is held after the difference is amplified, the S/N ratio of an obtained image signal can increase.
However, in the technique disclosed in Japanese Patent Laid-Open No. 2005-217771, when the amplification factor varies between a plurality of operational amplifiers (column amplification units), the levels of image signals output from these operational amplifiers may differ between them. Although this technique can reduce fixed pattern noise arising from variations of the characteristics between a plurality of pixels, the technique may not be able to reduce fixed pattern noise arising from variations of the amplification factor between a plurality of operational amplifiers. Especially at a high-sensitivity setting, streak noise may still stand out in an image obtained from an image signal.